Choosing the Right Service Lifecycle for Building a Scalable dotnet core application: Scoped, Singleton, or Transient?

In this content, I’m discussing based on the ecommerce application. When designing an e-commerce application, selecting the correct service lifetime (Scoped, Singleton, Transient) is critical for performance, scalability, and maintainability. Here’s how you can choose the appropriate service lifetime for various components of an e-commerce app:

1. Scoped Services

Use Case: Request-Specific or User-Specific Logic

Services that are bound to the lifecycle of a single HTTP request should use the Scoped lifetime. These include:

Examples

1. User Cart Service

• • Reason: Each user’s cart is unique for their session and request. You don’t want different users or requests to share the same cart state.

builder.Services.AddScoped<IOrderProcessingService, OrderProcessingService>();

builder.Services.AddScoped<IOrderProcessingService, OrderProcessingService>();

3. Unit of Work or Repository

• • Reason: Database operations are tied to a specific request, and the database context must be consistent across a single request.

builder.Services.AddScoped<IUnitOfWork, UnitOfWork>();
builder.Services.AddScoped<IProductRepository, ProductRepository>();

2. Singleton Services

Use Case: Shared Data or State Across the Application

Services that are global and shared across requests should use the Singleton lifetime.

Examples

1. Application Configuration Service

• • Reason: Application-wide settings (e.g., app name, payment gateway keys) are typically immutable and don’t require frequent instantiation.

builder.Services.AddSingleton<IAppConfigService, AppConfigService>();

2. Caching Service

• • Reason: Cache holds shared data (like product details or category listings). A singleton ensures the same instance is reused, improving performance.

builder.Services.AddSingleton<ICacheService, MemoryCacheService>();

3. Logging Service

• • Reason: Logging is a shared operation. Using a singleton ensures consistent log entries and avoids redundant instantiation.

builder.Services.AddSingleton<ILogger, FileLogger>();

4. Payment Gateway Client

• • Reason: Payment gateways often involve external API clients that should be initialized once and reused to reduce overhead.

builder.Services.AddSingleton<IPaymentGatewayClient, StripeClient>();

3. Transient Services

Use Case: Lightweight, Stateless, or Per-Use Logic

Services that are short-lived and used only briefly during their operation should use the Transient lifetime.

Examples

1. Email Notification Service

• • Reason: Sending emails is a stateless operation. A new instance can be created whenever needed without retaining state.

builder.Services.AddTransient<IEmailService, SmtpEmailService>();

2. Helper/Utility Services

• • Reason: Services like formatting currency or generating random order IDs are lightweight and don’t require state retention.

builder.Services.AddTransient<IFormatter, CurrencyFormatter>();

3. Data Transformation or Mapping Services

• • Reason: Services for converting entities to DTOs or performing data transformations should be short-lived.

builder.Services.AddTransient<IMapperService, AutoMapperService>();

Summary Table

Why Choose These Lifetimes?

1. Scoped:
   • Prevents unintended sharing of services tied to requests, ensuring correctness in multi-user environments.
2. Singleton:
   • Optimizes resource usage for shared, application-wide services.
   • Avoids the overhead of re-instantiating shared objects.
3. Transient:
   • Keeps lightweight services stateless and ensures they are disposed of quickly after use.

Real-Life Scenario: Checkout Process

When a user checks out:

1. Scoped:
   • The CartService keeps track of the user’s cart during the request.
   • The OrderService ensures all data for the current order is consistent.
2. Singleton:
   • The PaymentGatewayClient processes the payment using a single shared instance.
   • The AppConfigService provides gateway credentials without reloading.
3. Transient:
   • An EmailService sends the order confirmation email after checkout.
   • A Formatter formats the order details for the email body.

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